1. Field of the Invention
This invention is a technique for producing clad optical fibers with diameter variations in the core material but with uniform overall diameter. Such fibers exhibit enhanced mode mixing properties and hence reduced mode dispersion, yet do not have the disadvantages associated with fibers having overall diameter variations.
2. Description of the Prior Art
The obvious advantages of optical transmission have resulted in significant efforts to develop this field of technology; despite the fact that the optical signal must still be transformed to an electronic one for conventional electronic processing. The overriding advantages of optical communications include larger bandwidth and significantly smaller cable size. In conjested urban areas where underground duct room is rapidly being depleted, optical transmission provides a means for increasing capacity within the given physical area available.
While the transition to optical communication systems is inevitable, significant hurdles remain to be overcome. For example, the desire to transmit information in digital form, when combined with the economic pressures which favor the use of multimode fibers, places severe restrictions on the amount of mode dispersion which can be tolerated. This application is addressed to an improved technique for the fabrication of optical fibers with reduced mode dispersion.
The mode dispersion effect may be most readily understood in terms of the various possible paths by which a given light ray may traverse an optical fiber. The ray, for example, may be transmitted directly down the center of the fiber, or may reflect off the fiber walls any given number of times. Each of these possible paths has associated with it a different path length and hence a different traversal time. In a multimode fiber which can simultaneously support a multitude of modes, the width of a given light pulse is increased due to this nonuniform traversal time. Similar effects are associated with material dispersion phenomena but, at least for narrow wavelength sources, are overshadowed in severity by the above-described mode dispersion.
Initial attempts directed towards limiting mode dispersion involved the fabrication of single mode fibers. Since such fibers can support only a given single mode, there is clearly no pulse width degradation associated with different traversal times of different modes. Only one mode may traverse the fiber and it has a well defined traversal time. However, difficulties in launching light into such fibers, and a desire to use incoherent light for which such single mode fibers are inefficient, restrict the applicability of single mode fibers.
In multimode fibers the mode dispersion effect may be reduced by tailoring the material composition cross-section so that long path length modes have higher velocities. In such a fiber, all modes have approximately the same traversal times. A technique for fabricating such fibers is disclosed in U.S. Pat. Nos. 3,823,995 and 3,826,560.
In an article by S. D. Personic in the Bell System Technical Journal, Vol 50, No. 3, Mar. 7, 1971, page 843, an alternative technique for alleviating mode dispersion effects is suggested. Personic shows that while the pulse broadening associated with mode dispersion increases proportionately with the length of the fiber, efficient intentional mode conversion results in a broadening effect which is proportional only to the square root of the fiber length. Stimulated by this finding, numerous studies were made to determine the most effective technique for enhancing mode conversion. One particular method involves the introduction of longitudinal gradations in the index of refraction of the fiber. It has been found that to maximize mode mixing while maintaining radiation loss mechanisms within tolerable limits, the spatial periods of such gradations in the fiber must be approximately between 1 and 10 millimeters for transmitted light in the visible and near visible regions of the spectrum. Further reduction in radiative losses may be realized by introducing radial, longitudinal and azimuthal variations in index of refraction as discussed in U.S. Pat. No. 3,909,110.
In U.S. Pat. Nos. 3,666,348 and 3,687,514, it is shown that the introduction of diameter variations in the optical fiber transmitting core accomplishes the same efficient mode conversion as do longitudinal variations in the index of refraction. In a commonly assigned U.S. Pat. No. 3,912,478, a technique is presented for introducing such diameter variations after the drawing process. However, the fiber drawn according to the teachings in that patent either has overall diameter variations or must be subsequently clad.